Nested Span in TLF!

Hi folks...

There is problem here:

I have something like this in my RichEditableText's textFlow:

str = "<TextFlow xmlns='http://ns.adobe.com/textLayout/2008'>"+"<div><tab/>" +
      "<span fontFamily='tahoma' fontSize='12' color='0xc08057'>text specific word text</span></div>"+
      "</TextFlow>";
RichEditableText.textFlow=TextConverter.importToFlow(str, TextConverter.TEXT_LAYOUT_FORMAT);

I want to use replace() func, just like this:

var myPattern:RegExp = new RegExp(TextInput.text,"gi")
str = str.replace(myPattern,"<span backgroundColor='#c17e56' color='#dec59d'>"+TextInput.text+"</span>");                    
RichEditableText.textFlow= TextConverter.importToFlow(str, TextConverter.TEXT_LAYOUT_FORMAT);

But after doing this, my specific word that I had been written in my TextInput, would'nt be appear inside of my text. I think because of another <span> in the main text. the result is something like this:

<span fontFamily='tahoma' fontSize='12' color='0xc08057'>
     text <span backgroundColor='#c17e56' color='#dec59d'>specific word</span> text
</span>

how can I fix this?!